The invention relates to an improved optical member such as a transparent, planar, curved or shaped member or glass sheet having at least one layer or coating that provides useful, improved washability or cleanability properties. The coatings of the invention provide a surface composition and a shaped nanostructure that promotes removal of particulate soils when the surface and soil are contacted with water or aqueous cleaning solution. More particularly, the invention relates to an optical member used in a window pane, window light, window glass, wind screen, electronic display, wind shield or other substantially planar transparent member used in any structure, conveyance, instrument, device, etc., using a transparent member to permit viewing through or across a boundary.
Optical or transparent members are made of materials that permit transmittal of light in a manner that does not substantially distort an image. Such images include an aspect or environmental scene, an interior setting, an incandescent or florescent image, etc. Transparent members are typically made of non-crystalline materials used above the glass transition temperature. Transparent materials include inorganic glasses such as silicate glass, silicate-soda ash glass, borosilicate glass, etc.; thermoplastics such as polycarbonate, acrylic, etc. and other specialty crystalline and glassy materials.
The most common transparent members comprise silicate, and silicate-soda ash glass. Such glass technology has evolved since antiquity. These glass materials are typically understood to be an inorganic substance in a highly thickened but xe2x80x9cliquidxe2x80x9d state of the substance. As a result of a reversible change in viscosity, such materials attain such a high degree of viscosity to be, for all practical purposes (in a 40+ year useful life) rigid and non-flowing. Common silicate-soda ash window glass is manufactured from commonly available silicate (SiO2) minerals and carbonate (Na2CO3) minerals. The basic structure of silicate glass is the silicon-oxygen tetrahedron in which a silicon atom is in an sp3 tetrahedral bonding structure coordinated to four surrounding oxygen atoms. The oxygen shared between tetrahedron are called bridging oxygens. Virtually all such glass compositions comprise silicate glasses containing modifiers and intermediates. The addition of a modifier such as sodium oxide, boron compounds or sodium carbonate to the silica network alters the structure cleaving Sixe2x80x94Oxe2x80x94Si bonds to form an Sixe2x80x94Oxe2x80x94Na+ or other modified linkage. Examples of chemicals that have been used to improve the physical nature of the glass layer include alkaline earth metal compounds; boric oxide compounds; alumino-silicate glass generating compounds; lead compounds; borate and phosphate glass compounds; oxides including germanium, arsenic, antimony oxides, etc,; sulfur, selenium and tellurium compounds; and halogens such as zinc chloride, and BeF2 are also known. The purpose of these chemical modifications to the glass composition improves the mechanical properties such as hardness, the chemical stability, the heat resistance, or other physical or optical properties of the glass relating to end use requirements.
Most silica glass currently manufactured results from a process in which raw materials are converted at very high temperatures to an homogeneous flowable melt. The melt results from heating a combination of one or more typical ingredients such as glass sand (SiO2), soda ash (sodium carbonate), limestone (CaCO3), feldspar or other inorganic oxides such as potassium oxide, magnesium oxide, zinc oxide, barium oxide, lead oxide, etc. The inorganic materials are blended and melted at high temperatures typically from about 1500xc2x0 C. to 1800xc2x0 C. forming a flowable melt. The melt is then drawn from the heater and is drawn, rolled or quenched depending on the desired shape and end use. Bottles, dishes, optical lenses, tubes, sheets, cylinders, etc. are formed by floating, blowing, pressing, casting or spinning the glass to cool the glass to a solid. Large glass sheets are typically manufactured by floating the melt on molten tin in a non-oxidizing or reducing environment to form a planar extremely flat glass sheet with parallel faces. The glass face contacting the tin bath tends to acquire an amount of tin oxide (SnO2) on the glass that typically range in trace amounts on the glass sheet. Such tin residues do not comprise any nanostructure regions but are only a random surface scattering of tin oxide. These chemically modified glasses typically enhance the macro thermal, electrical and mechanical properties of the gross material.
The formation of association of one or more functional layers with one or more transparent layers of an optical member or glass sheet is also known. Mirrored layers have been made since antiquity. The association of a macro polymeric layer with one or more glass sheets is also known, for example, Safety glass in automobile manufacture comprises a sandwich comprising two layers of glass with an intermediate polyvinylbutyral layer. Optical members such as glass sheets have been surface modified using various chemical deposition techniques to form organic and inorganic layers on the glass. Such layers have been combined with organic silicone compounds, organic film forming materials, surface derivatizing organic materials, olefinic polymeric forming compositions and other materials that form macro layers on the glass surface. The formation of inorganic coatings on glass sheets is also commonly performed during glass manufacture. At high temperature, glass sheets tend to favorably react with organic and inorganic materials to form active macro coatings on the glass. Kirkbride et al., U.S. Pat. No. 4,019,887; Landau, U.S. Pat. No. 4,188,444; Shibata et al., U.S. Pat. No. 5,304,399; and others show the formation of a silicon or silica complex from continuous chemical treatment of the hot glass substrate with a non-oxidizing reactive silane containing compound. The formation of other simple macro layers using such deposition techniques is well within the skill of the ordinary artisan in this technology area. These relatively simple macro coatings typically improve the mechanical, chemical and thermal resistance of the glass surface to conditions in its use locus.
Coatings on optical members such as glass sheets having an improved geometry are also known. Ohwaki et al., U.S. Pat. No. 4,855,176, disclose macro structures (structures having millimeter size dimensions) with hydrophilic and hydrophobic regions to improve the anti-blurring properties of optical members used in windows, mirrors, etc. Similar to the technology shown in the Ohwaki et al. disclosure other patents relate to forming macro films on optical members that have varying degrees of tendency to associate with aqueous materials such as Komatsu, U.S. Pat. No. 5,594,585, which shows a hydrophilic film made from silicon dioxide. Sugawara et al., Japanese Application No. 07-33599, show a hydrophilic mirror coating comprising a metal oxide having a macro structure. Kai et al., Japanese Application No. 05-315261, show a hydrophilic mirror coating comprising silicon dioxide, zirconium dioxide, titanium dioxide, aluminum oxide and others to form a surface that rapidly drains incident water. Endo et al., Japanese Application No. 62-168702, show a hydrophilic transparent film made from indium oxide, tin oxide and others. Tiller et al., European Application No. 594171, disclose a SiOx coating using flame-pyrolytic deposition of an organo silane to form a hydrophilic surface.
The prior art taken as a whole focuses on forming chemical modified surface layers having thick layers or macro structures (dimension greater than 1 mm) for the purpose of improving chemical, thermal and physical resistance and to improve the hydrophilicity of the surface to improve visibility.
Self-cleaning glass technology is also known and are different in mechanism than improved cleaning materials. The improved cleaning technologies creates a structure that reduces the binding energy of the soil to the glass coatings. Self cleaning technology involves the manufacture of glass compositions or coatings that tend to absorb incident photons of visible light into surface layers and then convert such incident energy into an excited glass effect or excited surface coating effect that tends to energetically disassociate or desorb a soil particulate or layer from the surface. Currently, titanium dioxide (TiO2 containing layers) are being developed which can absorb typically ultraviolet light to increase self-cleaning properties. Self-cleaning technologies are described in xe2x80x9cLight-induced amphilic surfaces,xe2x80x9d R. Wang et al., NATURE, Vol. 388, (1997) p. 431; xe2x80x9cPhotogeneration of Highly Amphilic TiO2 Surfaces,xe2x80x9d R. Wang et al., Advanced Materials, Vol. 10, No. 2 (1998) pp. 135-138; xe2x80x9cPhoto-oxidately self-cleaning transparent titanium dioxide films on soda lime glass: The deleterious effect of sodium contamination and its prevention,xe2x80x9d Y. Paz et al., J. Mater. Res., Vol. 12, No. 10 (1997) pp. 2759-2766; and xe2x80x9cPhotooxidative self-cleaning transparent titanium dioxide films on glass,xe2x80x9d. Paz et al., J. Mater. Res., Vol. 10, No. 11 (1995) pp. 2842-2848.
Recently, significant interest has arisen regarding technologies that improve the washability or cleanability of glass surfaces. Washability or cleanability relates to the ease of removing a variety of soils including hydrophilic soils, hydrophobic soils, particulate soils, etc. from glass surfaces. Such properties are measured using known techniques. In our initial work in this area, we have found the technologies disclosed above provide no important improvement in cleanability or washability of the glass layers. We believe the simple macro modifications of the glass surface are not engineered to promote the removal of colloidal-sized particles of soil associated with the glass surface. We believe a substantial improvement in surface technology is required to result in substantial improvement in cleanability or washability of optical members such as glass, lights or sheets.
During the exposure of an optical member to its environment, the surface can acquire soil forming a residue. Large gross soils can readily be removed simply with a water jet or spray. Difficult to remove soils typically comprise relatively small particulate compositions that become closely associated with a glass surface. Such particulate materials arise as a collection of particulate. Each particle is typically colloidal in size and can have a dimension that ranges from about 100xc3x9710xe2x88x929 meters to about 100xc3x9710xe2x88x927 meters (100 to 10,000 nm). We believe such particles have a nature, or attain a surface charge, that causes a strong binding or association of the particulate to the glass surface that, in combination with normal VanderWalls forces results in an association with the glass surface that results in a hard to remove soil contamination. We have found that the strong association of such soils with optical member surfaces can be substantially weakened and rendered more washable or cleanable using a nanostructure coating technology. In the nanostructure technology of the invention, an ordered or random surface structure having a nanostructure dimension of between 1 and 500 nanometers, preferably 2 to 100 nanometers (nanometers or nm, 10xe2x88x929 meters) having at least a hydrophic region or at least a hydrophobic region in the structure can cause a substantial reduction in the association or binding strength of the particulate soil with the glass surface. Typically the particle is more easily cleaned if it is residing on a nanostructure with a size substantially less than the particle. Both the size of the surface structure and the chemical nature of the surface reduces the binding strength or degree of association of the particulate to the surface. The surface structure or roughness on a nanoscale can reduce the binding association of the particle with the surface since such binding associations tend to be reduced by a factor of 1/r6 as the particulate is withdrawn from the surface. The structure or roughness of the surface tends to cause the particle to associate with a smaller or reduced quantity or fraction, less than about 10% of the nanostructure surface. The relatively large particle resides on a relatively small nanostructure. The majority of the particle surface tends to be remote from the nanostructure and contributes substantially less to its binding association because of the nature of the binding forces. Further, the structure, containing enhanced hydrophilicity or enhanced hydrophobicity, or both, tend to reduce the surface bonding association of the particulate with the surface since individual soil particles tend to be either hydrophilic or hydrophobic and tend to be repelled by surfaces with a different character. Accordingly, the creation of a nanostructure surface having a preferred geometry combined with a preferred chemical nature can substantially improve the cleanability and wettability of optical members when contacted with aqueous cleaning materials.
The soil materials that are the focus of the application are soil particulate materials that can comprise either hydrophilic or hydrophobic compositions. These particles have sufficient size such that they are substantially affected by gravity when suspended in the air. Such particles are of sufficient size to adhere by hydrophilic and hydrophobic properties or Vander Walls forces or electric charge mechanisms to the surface of an optical member. Particles smaller than about 200 nanometers are simply too small to cause any significant permanent association with a window surface and as a result typically do not form a soil deposit. Particles of significant size, i.e. greater than about 100 microns, typically are relatively easily removed because of size and weight. Particles of intermediate size, 200 nm to 100 microns, particularly particles that can support a significant charge density are made of materials with substantial hydropobicity or hydrphilicity can cause a tenacious binding or association with the glass surface resulting in a difficult to remove soil. Collections of colloidal particles typically having a particle size that range from about 100 nanometers to about 10 microns tend to include the most tenacious and most difficult to remove soil deposits. It is this type of particulate soil that is of greatest concern to washing or cleaning the optical member surfaces of the invention. For the purpose of this application, the term xe2x80x9cmacroxe2x80x9d tends to relate to structures having a minor dimension that is typically greater than about 1 millimeter. The term xe2x80x9cmicroxe2x80x9d is intended to refer to structures having a minor dimension that is typically greater than about 1 micrometer (10xe2x88x926 meters). The term xe2x80x9cnanostructurexe2x80x9d typically refers to structures having a minor dimension that is greater than about 1 nanometer but typically substantially less than about 800 nanometers, often about 10 to 500 nm, preferably about 50 to 300 nanometers.